This invention relates to a method for producing organic light-emitting devices (OLEDs).
Organic light-emitting devices such as described in U.S. Pat. No. 5,247,190 or in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference, have great potential for use in various display applications. According to one method, an OLED is fabricated by coating a glass or plastic substrate with a transparent first electrode (anode) such as indium tin oxide (ITO). At least one layer of a thin film of an electroluminescent organic material is then deposited prior to a final layer which is a film of a second electrode (cathode), typically a metal or alloy.
A typical cathode layer will comprise a layer of a low work function metal or an alloy containing at least one low work function metal.
Such a cathode layer is typically deposited by vacuum evaporation or by a sputtering technique such as rf sputtering or dc magnetron sputtering. When the underlying organic layer is a layer of a relatively sensitive material such as a soluble conjugated polymer, vacuum evaporation is often the preferred technique for depositing the first layer because it is a relatively low-energy process which causes less damage to the underlying layer of organic material.
It has been noticed by the inventors of the present invention that cathode layers deposited by conventional vacuum evaporation techniques contain pinholes, through which water and oxygen are able to enter the device and initiate reactions at the interface between the organic layer and the cathode layer. These reactions result in the formation of non-emitting black spots with a consequent degradation in device performance.
It is an aim of the present invention to provide a method for forming an electrode on a light-emissive organic material, which avoids or at least reduces the problems of the prior art.
It is an aim of the present invention to reduce black spots in an organic light-emitting device.
Vacuum evaporation and sputtering are the conventional techniques used for depositing a thin layer of a conductive material on to the electroluminescent layer to form the cathode. Vacuum evaporation is often the preferred technique (particularly where the organic material on to which the conductive material is to be deposited is a relatively sensitive material such as a soluble conjugated polymer) because it causes less damage to the underlying organic material than sputtering.
However, with conventional methods of vacuum evaporation, there has been the problem that the deposited film contains pinhole defects, as discussed in SID Digest F-28, 1997, Arjan Berntsen et al. Water and oxygen are able to enter the device via such pinholes in the evaporated cathode layer and initiate reactions at the interface between the organic layer and the cathode layer. These reactions result in the formation of non-emitting black spots with a consequent degradation in device performance.
It is an another aim of the present invention to provide a method for depositing a conductive layer on an organic substrate by evaporation, which avoids or at least reduces the problems of the prior art.
The inventors of the present invention found that the formation of a conformably deposited layer on top of a layer having undesirable pinholes has the effect of reducing black spots in the resulting light-emitting device.
According to a first aspect of the present invention, there is provided a method for reducing black spots in a light-emitting device comprising a light-emissive material interposed between a first electrode and a second electrode such that the first and second electrodes are capable of injecting charge carriers into the light-emissive organic material, the method comprising: forming at least one of the first and second electrodes by depositing onto the light-emissive organic material a first layer of material by a deposition technique which intrinsically results in undesirable pinholes; and depositing a second layer of material onto the layer of first material by a conformable deposition technique, such as a sputtering technique, which forms a layer over the whole surface of the target substrate, regardless of the existence of portions of the substrate which are not directly exposed to the deposition source.
In a preferred embodiment, the first layer, and preferably also the second layer, are made of materials which are capable of injecting negative charge carriers into the light-emissive organic material. The first layer is preferably comprised of a metal having a work function of 3.7 eV or less, preferably 3.2 eV or less, or an alloy containing such a low work function metal.
This will be particularly advantageous if both the first and second layers are composed of materials which are capable of injecting charge carriers (negative charge carriers in the case of a cathode) since substantially the entire surface of the light-emissive organic material underlying the respective electrode will be available for injection thereinto of the respective charge carriers.
According to a second aspect of the present invention, there is provided a method for forming an electrode on a substrate comprising a layer of organic material comprising the steps of: depositing one or more layers of conductive material onto the layer of organic material by a low energy technique such as a thermal evaporation technique to form an intermediate product; and depositing a layer onto the surface of the outermost low energy deposited layer of the intermediate product remote from the layer of organic material by a conformable deposition technique; wherein the outermost low energy deposited layer and the conformably deposited layer are deposited without exposing the intermediate product to any atmosphere other than those used to deposit the outermost low energy deposited layer and the conformably deposited layer.
The low energy technique can be any deposition technique that does not cause undue damage to the underlying organic layers, such as a thermal evaporation technique.
The outermost low energy deposited layer and the conformably deposited layer are preferably deposited in a single chamber.
In one embodiment of the second aspect of the present invention, the organic material is a light-emissive organic material, and the low energy deposited layer adjacent the light-emissive organic material is a layer of a metal having a low work function, preferably calcium. The conformably deposited layer may, for example, be a layer of a ductile metal such as aluminium to serve as a contact. Alternatively, the sputtered layer may be a layer of a dielectric material such as aluminium nitride to serve as a protective layer.
When more than one layer is deposited on the organic layer by a vacuum evaporation technique prior to the deposition of the conformably deposited layer, it is preferable that each such evaporated layer is deposited without breaking the vacuum between the deposition of each such evaporated layer, and further preferable that each such evaporated layer is deposited in the same chamber as the outermost evaporated layer and the conformably deposited layer.
RF sputtering or DC magnetron sputtering may, for example, be used as the conformable deposition technique. In the case that a layer of a metal or metal alloy is to be deposited by sputtering, the sputter target/cathode is made out of the metal or metal alloy and an inert gas such as argon or neon is used as the discharge gas. Neon is preferred to argon, because sputtering with neon causes less damage to the underlying organic layer via the pinholes existing in the evaporated layer upon which the sputtered layer is deposited.
According to a third aspect of the present invention, there is provided a use in the production of an organic light-emitting device of an evaporation technique comprising depositing a layer of material from an evaporation source onto a surface of a substrate by vacuum evaporation whilst moving the substrate so as to vary the angle of orientation of the surface of the substrate with respect to a fixed point of the evaporation source.
According to a fourth aspect of the present invention, there is provided a method of producing an organic light-emitting device comprising the steps of providing a layer of light-emissive organic material on a first electrode, and providing a second electrode on the layer of light-emissive organic material, wherein the formation of the second electrode comprises the step of depositing a layer of material from an evaporation source on to the surface of the layer of light-emissive organic material remote from the first electrode by evaporation whilst moving the layer of light-emissive organic material so as to vary the angle of orientation of the surface thereof with respect to a fixed point of the evaporation source.
According to a fifth aspect of the present invention, there is provided a method of depositing a substantially pinhole-free electrode layer onto a substrate comprising a light-emissive organic layer, said method comprising depositing the material of the electrode layer from an evaporation source onto a surface of the substrate by evaporation whilst moving the substrate so as to vary the angle of orientation of the surface of the substrate with respect to a fixed point of the evaporation source.
In another embodiment of the third to fifth aspects of the present invention, the substrate is also rotated about a second axis orthogonal to the first axis. In one embodiment, this second axis is orthogonal to the plane of the substrate. The rotation about each of the first and second axes is preferably carried out simultaneously.
The method according to the third to fifth aspects of the present invention is particularly applicable to the deposition of a conductive layer directly on the light-emissive organic layer. It is also equally applicable, for example, to the deposition of a second conductive layer on the surface of a first conductive layer which has been deposited on the light-emissive organic layer by, for example, a conventional vacuum evaporation method or by the method of the present invention.